Electrochromic devices have been increasingly utilized for actively controlling the transmissivity of glazing and other products. Compared to other methods of varying transmissivity, such as chemically darkening or photochromic glass, the electrochromic device has several advantages including the ability to adjust transmissivity according to user preference, a relatively rapid response time and a greater range of transmissivity. Electrochromic devices can be deposited on plastic film which increases the range of applications, including retrofitting. They do not require a constant source of power, other than current to make up for internal leakage, to maintain the device in any darkened state. For these reasons, electrochromic devices are seen as the most promising technology for future variable light transmission products such as windows, car windshields, car mirrors and eyeglasses.
An electrochromic device is often in the form of a series of layers deposited on a transparent substrate, although for mirrors the substrate does not have to be transparent. The functional electrochromic layers typically include a cathodic layer that takes on color as ions are intercalated into the layer, and an electron source layer located near the cathodic layer. The functional electrochromic layers are sandwiched between two transparent conductive layers, one of which is typically applied directly to the substrate.
In use, an electrical potential is applied between the two transparent conductive layers, causing a current to flow between the functional electrochromic layers. As the current flows, ions are depleted from the electron source layer and intercalated into the electrochromic material layer, causing it to darken. Most electrochromic devices have some amount of internal leakage, whereby electrons return to the electron source layer, reducing the darkness of the electrochromic material. In a steady state for a given transmissivity level, current flowing through the functional electrochromic layers because of the potential between the transparent conductive layers is equal to the internal current leakage in the functional layers. At steady state, the transmissivity of the device remains unchanged.
One means for providing an electrical current to an electrochromic device is the use of a photovoltaic cell. Because the photovoltaic cell provides electric current in response to light incident on the cell, the photovoltaic cell may be used both to power and to control the electrochromic window.
One configuration powering an electrochromic device with a photovoltaic cell is disclosed in U.S. Pat. No. 5,384,653, to Benson et al. In that configuration, an electrochromic layer is applied in one region of an inner surface of an insulated glass unit, and a photovoltaic cell array is applied in another area. As used herein, the term "insulated glass unit" refers to a multilayer unit having at least one sealed space between adjacent layers of glass. The photovoltaic cell is connected to the electrochromic layer through a polarity-reversing switch and a switch for switching between the photovoltaic cell and a battery. When a constant state of transmissivity is desired, the circuit is opened. The effects of internal current leakage, that slowly return the electrochromic layer to a transparent state, are ignored.
The polarity-reversing switch and the switch for switching between the photovoltaic cell and the battery in the '653 patent are housed in a control box remote from the window, and are connected to leads passing through the seal between two adjacent glass plates of the insulated glass unit. This is problematic because the useful lifetime of an insulated glass unit typically depends on the integrity of its seals. The typical insulated glass unit does not have penetrations in the seal for passing leads. The addition of electrical leads penetrating the seal therefore adds assembly cost and product reliability risk. Furthermore, the existence of an external, hard-wired control box presents serious installation problems to the architect and builder.
Photovoltaic cell arrays in electrochromic windows are typically positioned near the perimeter of an electrochromic window for aesthetic reasons. Such positioning is shown, for example, in the '653 patent. A shadow formed by framing around the glass will typically fall on the photovoltaic array, dramatically dropping its output. Because the shadowing changes with the time of day and season of the year, it may be impossible to position photovoltaic arrays to completely avoid such shadowing.
U.S. Pat. No. 5,377,037 to Branz et al. discloses an electrochromic/photovoltaic film for use on eyeglasses. The electrochromic layers and the photovoltaic layers are applied consecutively on a glass substrate. The layers are deposited in such a way that the incident light passes through the photovoltaic layers before reaching the electrochromic layers. A bleeder resistor is connected in series between the photovoltaic layer and the electrochromic layer. The resistor is connected in series between the photovoltaic device and the electrochromic device. The resister is selected for a desired response of the electrochromic device to variations in ambient light.
A disadvantage of a series resistor in such a system is that, as explained in more detail below, it is ineffective controlling an electrochromic device having negligible leakage. Even a small current from the photovoltaic cell will continue to darken an electrochromic device having negligible leakage. A series resistor must therefore have sufficient resistance to reduce current flow to nearly zero in order to maintain a steady state transmissivity in an electrochromic device having negligible leakage. Such large resistance values create unacceptable response times to change the electrochromic device from transparent to opaque.
Electrochromic technology has been used in the glazing of both windows and skylights. Manual control of the devices, even when powered and controlled by a photovoltaic cell, is often necessary to meet the particular needs of the occupants of a given room. Benson et al. discloses the use of a control box connected to the window by wire. As noted, this presents difficulties in reliability and installation. Additionally, in the case of skylights, added problems are presented by the necessity of routing the wires to a control box accessible to the room occupants.
Thus, despite substantial efforts devoted heretofore to the problems associated with the installation, powering and control of electrochromic devices, there are substantial, unmet needs for improvements in such components.